Because of the harm it causes, and because of government regulations, numerous attempts have been made to capture mercury prior to its release from coal-fired power plants.
For example, Oehr, in U.S. Pat. No. 6,808,692, the contents of which is hereby incorporated in its entirety by reference, describes the use of molecular halogens such as chlorine gas (Cl2) to convert elemental mercury (Hg0) to mercuric chloride (HgCl2). Such a conversion is desirable because mercuric chloride is absorbable by alkaline solids, especially solids containing calcium oxide (CaO) or calcium hydroxide (Ca(OH)2). Typically, such collection is achieved ahead of particulate collection devices such as baghouses or electrostatic precipitators. Oehr's technique, however, is not equally successful for all types of coal.
For example, Oehr's technique was not completely effective for mercury removal when using 12 to 25 ppmv molecular chlorine injection into the flue gas obtained from full combustion of subbituminous and lignite coals. Such coals generate fly ash on partial or full combustion which are alkaline as evidenced by their ability to raise pH of their water-to-fly ash mixtures to values above 7. The applicants consider various ranks and forms of lignite and subbituminous coals to be coal for the purposes of this invention. Lignite and subbituminous coals represent almost half of Canadian and United States coal combustion in power plants. Other techniques for reducing mercury release from the burning of lignite coals are similarly ineffective or have shortcomings.
For example, Pennline et al., in U.S. Pat. No. 6,521,021, the contents of which is hereby incorporated in its entirety by reference, describe a Thief process, which includes the use of partially combusted coal solids to effect mercury removal from flue gas ahead of particulate collection devices such as baghouses or electrostatic precipitators. Applicants have examined Pennline et al.'s technique and found it to have deficiencies for mercury removal when using partially combusted lignite and subbituminous coal solids. Because lignite and subbituminous coals represent such an important energy source, and because reducing the amount of mercury released into the environment is so important, applicants have developed a method and apparatus for decreasing the amount of mercury released into the environment from combusting lignite or subbituminous coals is needed.
Nelson in U.S. Pat. No. 6,953,494 describes the use of a brominated “carbonaceous” substrate to effect mercury control combustion flue gas. This invention suffers from the following serious disadvantage: carbon, especially expensive powdered activated carbon, represents the majority component of the “carbonaceous” material e.g. “preferably powdered activated carbon” (page 6 lines 10-11). Also see also page 7, lines 13-14 “The important features of the sorbent substrate material are that it is significantly composed of carbon”.
Excessive use of carbon is undesirable due to contamination of resulting fly ash in blended cement applications especially outdoor winter applications due to freeze-thaw characteristics or undesirable coloring of the blended cement.